1. Field of the Invention
The present invention relates to a chemical solution coating method and apparatus and, more particularly, to a method and an apparatus for coating the back face of a semiconductor substrate used for a solar battery with a material for forming a diffusion preventing film.
2. Description of the Related Art
As a method of coating a semiconductor substrate with a chemical solution such as a resist, there has been used a method of dropping a chemical solution to be applied onto a semiconductor substrate and rotating the substrate at high speed, thereby coating the entire surface of the substrate with the chemical solution (hereinafter, referred to as “spin coating”) (see, for example, Japanese Unexamined Patent Publication No. Sho 62(1987)-121670).
In such spin coating, the semiconductor substrate is held on a rotating stage by suction and the rotating stage is rotated at high speed, thereby spreading the chemical solution dropped on the surface of the substrate to the entire surface of the substrate.
In the case where the chemical solution is dropped on a center portion of the substrate, the chemical solution spreads from the center portion toward the peripheral portion by the centrifugal force of rotation, so that the chemical solution is applied to the entire surface of the substrate. However, part of the chemical solution is splattered from the peripheral portion of the substrate to the outside due to the centrifugal force of rotation.
There has been also proposed a chemical solution coating apparatus in which a blocking member is provided at a position close to the peripheral portion of the back face of a substrate in order to prevent a resist from spreading to the back face of the substrate in the case of coating the surface of the semiconductor substrate with a resist by using such spin coating (see Japanese Unexamined Patent Publication No. 2001-110714).
There has been also proposed a chemical solution coating method in which, in the case of applying a resist by spin coating, the pressure in a lower region of the back face opposite to a coated surface of a substrate is made high and air current is generated from the lower region to the outside of the peripheral portion of the substrate in order to prevent the back face of the substrate from getting dirty by being adhered with a resist (see Japanese Unexamined Patent Publication No. 2000-49075).
A dopant solution (for example, a solution containing P2O5 and the like) is applied to one side of a semiconductor substrate used for a solar battery by using spin coating similar to the above spin coating.
In the case of applying a dopant solution (chemical solution 103) by spin coating, as shown in FIG. 6A, by rotating a substrate 102 held on a rotating stage 100 by suction, the chemical solution 103 dropped from a nozzle 101 is applied to the entire surface of the substrate 102. As shown in FIG. 6B, however, a dopant solution 103′ spreads along the side face of the peripheral portion of the substrate and even to a back face 104 of the substrate.
Generally, an electrode is formed on each of the surface and the back face of a substrate of a solar battery and current generated between the electrode is taken. It is necessary to prevent the electrodes from being electrically connected to each other.
As shown in FIG. 6B, if the dopant solution 103 spreads to the back face 104, it may cause poor insulation of the electrodes. Consequently, various devises have been proposed (see, for example, Japanese Patent Publication No. Hei 06(1994)-82853).
In Japanese Patent Publication No. Hei 06(1994)-82853, in order to prevent a PN junction layer formed on the surface from being formed on the back face, a mask solution application film is formed on the back face 104.
A masking solution 105 is used to prevent the dopant solution 103 applied on the surface from spreading to the back face, and plays the role of a diffusion preventing film. It is unnecessary to apply the masking solution 105 to the entire back face but it is sufficient to apply the masking solution along the sides of the back face in a strip shape. In the case of applying the masking solution only around the periphery of the back face, the position of dropping the chemical solution may be set to a position deviated from the center of the substrate to the periphery.
In the case of an angular substrate 102 whose shape is a quadrangle or the like as shown in FIG. 7, when the substrate 102 is held on the rotating stage 100 by suction for rotation, the masking solution is applied to an application region shown by the double-headed arrow. However, the masking solution 105 is applied also to regions to which the masking solution 105 does not have to be applied, so that the masking solution 105 is wasted. For example, in FIG. 7, it is unnecessary to apply the masking solution 105 to such large corner areas of the quadrangle. It is sufficient to apply the mask solution 105 only to areas each having a predetermined width (for example, a few mm) from each of the sides of the quadrangle.
The conventional chemical solution coating method using spin coating needs an apparatus for rotating a substrate at high speed and, further, needs a vacuum mechanism for closely attaching a rotating stage and a substrate. However, in the case where the surface of the substrate to be held by suction is rough, holding by suction is insufficient and there is a case that the substrate drops at the time of high-speed rotation. In particular, the substrate used for a solar battery is very thin like about 300 μm. Consequently, even the surface held by suction is slightly rough, holding by suction becomes insufficient and, in many cases, the substrate drops.
When the suction force is increased, however, since the substrate is very thin, the substrate cracks at the time of high-speed rotation. For the substrate for a solar battery, the suction force cannot be increased more than necessary.
In the spin coating, the substrate is rotated at high speed. Therefore, splash of the dropped chemical solution from the peripheral portion of the substrate to the outside cannot be avoided, so that a splash preventing cover or the like has to be attached.
Moreover, the splashed chemical solution is wasted, so that a chemical solution collecting mechanism may be provided on the outside but it is impossible to collect the solution perfectly. It is also difficult to prevent the chemical solution from being spread to the back face. Therefore, the amount of the chemical solution to be used cannot be reduced so much.